The flexible wedge gate valve shown in FIG. 1A (prior art) is the most commonly used isolation valve in critical service applications in nuclear and fossil fuel electric power generation plants and in petrochemical plants where reliable sealing under high temperature is an important requirement. In safety-related systems, these valves are operated by an electric motor actuator, and the valve and actuator assembly is commonly referred to as "motor-operated valve" (MOV).
The closing function of the MOV is achieved by moving the wedge into a seated position that blocks the fluid flow. Motion imparted to the stem by the actuator imparts the necessary opening and closing movement to the wedge. During the opening and closing strokes the motion of the wedge is guided by rails attached to the body which cooperate with slots incorporated in the wedge to constrain the wedge to a linear path and support the load imparted to the wedge by the pressure and flow of the process fluid.
The flexible wedge gate valve provides a metal-to-metal seal between the faces of the seats (which are typically welded to the body in critical service applications) and the seating faces on the wedge, which is also commonly called a "disk" or "gate". The total wedge angle between the two disk faces is relatively small and typically ranges from 6.degree. to 15.degree. in the industry. This relatively small wedge angle provides mechanical advantage and converts the externally applied stem force to a normal force at the seating faces which is much larger than the applied stem force, providing a tight seal across the disk under both high and low differential pressure applications. Compared to solid wedge gate valve designs in which the disk is too stiff to accommodate seat face distortion without adversely affecting sealing ability, the conventional flexible wedge design provides improved sealing capabilities by better accommodating seat face distortion resulting from changes in operating pressures, temperatures, and externally applied forces/moments to the valve ends. Because of its ability to provide a better seal than the solid wedge gates, the flexible wedge gate design has become the most common choice in critical service applications.
In spite of the advantage of better sealing capabilities compared to solid wedge designs, there are several problems with flexible wedge gate valves that have not been overcome by present-day designs:
1. Recent full-scale blowdown tests on several flexible wedge gate valves show that opening or closing operation under high energy flow conditions can inflict severe damage to the disk guide ear elements, body guide rails, and the downstream seat and disk faces, causing a substantial and unpredictable increase in the stem thrust required to operate the valve, which in extreme cases can prevent the valve actuator from fully opening or closing the valve. PA1 2. Closing the valve hot and allowing it cool down can result in a condition commonly referred to as "thermal binding" which can cause even the so-called "flexible" wedge to get stuck in the closed position due to differential temperature between the wedge and valve body at the time of closing and the resulting differential thermal contraction while cooling. The present day flexible wedge gate design evolved from the solid wedge design with the goal of overcoming thermal binding problems, which are most pronounced in solid wedge gate valves. The flexible wedge gate valve designs have not been totally successful in achieving the goal, as evidenced by a number of reports by the United States Nuclear Regulatory Commission (NRC) including NUREG-1275 Vol. 9 and other industry reports. It has been found that the opening thrust can increase substantially and unpredictably in current flexible wedge gate designs when subjected to unfavorable temperature changes. In extreme cases, motor burn-out, motor stall, and damage to the seat faces and stem to disk connection can occur when the electric motor actuator attempts to open a valve that is stuck in the closed position due to thermal binding. PA1 3. External forces and moments applied to the valve ends by the attached piping can cause the faces of the seats (which are welded to the body) to move closer together, distort, and become angularly misaligned with respect to each other. With the disk in the closed position, seat displacements resulting from external forces and moments can lead to "pinching" of the disk and create significant variations (increase or decrease) in the sealing contact force between the disk and seat faces. The current flexible wedge gate valve designs, especially those for high pressure service, do not have sufficient disk flexibility to accommodate such seat face displacements/distortions without a degradation of sealing ability and/or an unpredictable increase in thrust required to open the valve. PA1 4. It is the firm seating of the wedge against the inclined seat surfaces that provides the seat contact stress necessary to initiate sealing. Disk seating is controlled by torque sensitive switches of the valve operator mechanism, rather than by thrust or by actual disk position. Since the stem and disk friction realized in service can be significantly less than the conservative values used for determining the torque switch trip setting, the actual stem thrust can be significantly higher than actually required to wedge the valve and achieve a seal, therefore the gate is subject to being wedged deeper than desired, which can overload the disk and seats. Failure or improper setting of the torque switch can also result in overload of the disk and seats.
The above mentioned problems are discussed in more detail here below to provide an insight into the responsible mechanisms.
During valve operation under high energy flow conditions, conventional flexible wedge gate valve designs can suffer severe damage to the disk, disk guides, and the seats, as shown in the tests conducted by the NRC and reported in NUREG/CR-5406 and NUREG/CR-5558. With the disk in the intermediate travel position, the high flow velocities and high differential pressure across the disk causes high internal reactions at the sliding interfaces of the internal valve components. As the valve is opened or closed under these conditions, severe damage can occur at two distinct locations: (1) at the lower edge of the guide rail to disk guide slot interface, and (2) the disk and downstream seat interface at approximately four o'clock and eight o'clock positions.
The disk-to-guide rail clearances in flexible wedge gate valve designs offered by different manufacturers span a wide range, from as low as 0.020 to 0.040 inch (typically referred to as "tight guide clearance" designs) to as high as 0.20 to 0.30 inch (typically referred to as "large guide clearance" designs). In designs having tight guide clearances and/or long guides, the disk rides on the guides during almost the entire stroke and makes contact with the downstream seat face only in the last 5 to 10 percent of the stroke near the fully closed position. The high differential forces across the disk in the intermediate travel position are transmitted to the guides, resulting in very high peak contact stress at the lower edge of the disk guide slot. The contact stress is high enough to cause severe damage to the guide surfaces, causing a significant and unpredictable increase in the stem thrust required to operate the valve, which may prevent the valve from achieving its fully open or closed position. This type of damage has been found to occur in valves with a short guide length, which allows the disk to tip during travel, as well as in valves with long guide length, in which the disk rides flat on the guide.
In valves with short guides, in addition to guide damage, the disk can tip sufficiently to contact the downstream seat face in the intermediate travel position. This creates a two-point contact between the lower circular end of the disk face and the downstream seat face at approximately four o'clock and eight o'clock positions. Under high differential pressure conditions, the localized contact stress at these two points become extremely high, causing severe plastic deformation, gouging, and machining damage to both the disk and seat faces, which results in a significant and unpredictable increase in the stem thrust required to operate the valve. In extreme conditions, the force requirements become so high that the operator is unable to open or close the valve, thus leading to significant safety concerns in nuclear and fossil fuel power generation plants as well as petrochemical plants.
The thermal binding problems encountered with flexible wedge gate designs are most severe for high pressure valves. The conventional flexible wedge gate valve design uses a one-piece disk in which the pressure boundary plates are connected by a smaller diameter circular hub to provide flexibility. The pressure boundary plates are required to withstand the maximum anticipated differential pressure without exceeding applicable stress limits. In nuclear power plant applications, the disk is required to withstand faulted design pressure conditions, also referred to as maximum pressure or worse case conditions, while maintaining maximum stresses within the applicable code limits, e.g. the American Society of Mechanical Engineers (ASME) Section III Code. This faulted design pressure condition dictates the thickness of the disk pressure boundary plates. For high pressure valves (e.g., ANSI Class 600, 900, or higher), the pressure boundary plate thickness becomes so great that the so-called "flexible" wedge gate designs become extremely stiff (almost as stiff as the solid wedge designs), thus suffering from the same type of thermal problems as those encountered with solid wedge gate valves.